1. Field of the Invention
The present invention relates generally to methods of treating cellulosic-compound containing materials, and more specifically to methods of making cellulose-based materials more hydrophobic using a composition which esterifies available hydroxyl groups, where such a composition is useful in modifying/coating surfaces of cellulose-based materials including packaging products.
2. Background Information
Cellulosic materials have a wide range of applications in industry as bulking agents, absorbents, and printing components. Their employment is preferred to that of other sources of material for their high thermal stability, good oxygen barrier function, and chemical/mechanical resilience (see, e.g., Aulin et al., Cellulose (2010) 17:559-574). Of great relevance is also the fact that these materials are fully biodegradable once dispersed in the environment, and that they are totally nontoxic. Cellulose and derivatives are therefore the material of choice for environmentally friendly solutions in applications such as packaging for foodstuff and disposable goods.
The many advantages of cellulose are nonetheless countered by the hydrophilicity of the material, which shows a high affinity for water and can easily absorb large amounts of it (see, e.g., Aulin et al., Langmuir (2009) 25(13):7675-7685). While this is a benefit for applications such as absorbents and tissues, it becomes an issue when the safe packaging of watery material (e.g., foodstuffs) is required. Long term storage of food, especially ready made meals which contain a significant amount of water, is made problematic in cellulose trays as they would first become soggy and then ultimately disrupt under the hydration of their fibers.
This problem is usually addressed in the industry by coating the cellulose fiber with some kind of hydrophobic organic material, for example a resin or a polymer, which would physically shield the underlying hydrophilic cellulose from the water in the contents. Materials such as PVC are routinely used for this purpose and are physically attached (i.e., spray coated) on the surfaces to be treated.
A similar problem is encountered when sealing foodstuff in its container by means of a film. This film requires even more stringent properties than the container itself. On top of the resistance to mechanical stress, the film must be thin enough to be peeled off, should ideally be transparent, heat resistant, and impermeant to gases such as CO2 and oxygen, non toxic, and hydrophobic. Again, plastic in the form of polymers and resin is the present solution of the industry.
The need to come up with more environmentally friendly, as well as renewable, packaging solutions has cast a shadow over the use of plastic. New materials have been designed which are either derived from natural sources or semi-synthetic sources and therefore are renewable and/or biodegradable. Materials such as poly-lactic acid (PLA) and poly hydroxyalkanoate (PHA) are the present golden standard for biodegradable “plastics.” However, they suffer the drawbacks of heat instability and water sensitivity which severely limit their use in the packaging industry. It would be desirable to design a coating which is both heat resistant and hydrophobic, while maintaining the biodegradability requirement.